The rocks of the Birchdale-Indus area, in northern Minnesota, are part
of a poorly-exposed volcanic-sedimentary sequence within the southwest
extension of the Wabigoon Volcanic Belt of Canada. The bedrock is Early
Precambrian (or Archean) in age and includes mafic to intermediate lavas
and sub volcanic intrusive rocks; felsic dikes, agglomerate, tuff, and
volcaniclastic rocks; iron-formation and associated metasediments; and
granitic rocks of Algoman age. All rock types are cut by northwest-trending
Middle Precambrian mafic dikes. The area is largely covered by Pleistocene
glacial deposits.
The volcanic-sedimentary sequence has been isoclinally folded and has
steeply-dipping northeast-trending bedding and foliation. A doubly-plunging
anticline and a syncline have been mapped within the area, and a second
generation of folding on more northerly trending axes is suggested by some
structural observations. Three sets of faults and fractures have been
delineated. All the Lower Precambrian rocks were metamorphosed to
amphibolite grade during the Algoman event.
Synthesis of the geology of the Birchdale-Indus area with that of the
better exposed Emo area in adjacent Ontario (Fletcher and Irvine, 1954-)
provides a firm basis for interpretation of the development of the volcanic
accumulation. Mafic and intermediate volcanic and intrusive rocks
apparently constitute a lower stratigraphic unit. An explosive felsic
volcanic center, marked by abundant agglomerate, developed upon the older,
mafic platform in the vicinity of Birchdale, Indus, and Emo. Felsic tuff,
volcaniclastic rocks, and iron-formation were deposited outward from this
center.
The Birchdale-Indus area and areas to the west and south have been
actively explored for base metal sulfide deposits during the past decade.
Thick zones of massive, submassive, and disseminated pyrite and/or
pyrrhotite have been penetrated at several localities, but copper and .zinc
have not been found in economic quantities. Several of the iron-sulfide
bodies that were penetrated are associated with oxide iron-formation. To
assist in further exploration, geophysical and geochemical anomalies have
been located by ground survey methods by the Minnesota Department of
Natural Resources.